Power companies' efforts to reduce sulfur dioxide and other emissions have focused largely on the use of advanced emission control equipment and improved operating practices. A number of different AQC Systems have evolved for flue gas cleaning including Baghouses, Dry Scrubbers and selective catalytic reduction (SCR) devices. In dry scrubbers, a reagent is introduced into a reactor where it mixes with the flue gas stream. The gaseous pollutants react with the chemicals in the reagent and are captured in a dry solid form that is then removed by the downstream particulate collector. These scrubbers can reduce acid gas emissions by more than 95 percent. Some scrubbers as well as other pollution control devices, such as cyclones, require a minimum gas velocity in order to operate properly. This is not a problem when a power plant is operating at high-load conditions because the flue gas flow rate can support the operation of the pollution control equipment. However, it may become a problem during low load conditions and may cause the AQC System to shut down. With the advent of new air pollution control regulations that do not permit any periods of non-compliance, even during boiler start-up and shutdown, taking the AQC System offline at low load is no longer an option. This has driven pollution control system manufacturers to add Flue Gas Recirculation (FGR) systems to their equipment in order to maintain the flow through the AQC System above the minimum operating velocity during all boiler load conditions.
The FGR system recirculates a portion of the treated flue gas back into the inlet of pollution control equipment, thereby increasing its volumetric throughput to a level at or above the minimum required to operate the equipment. The FGR system typically consists of a duct connecting the discharge side of the ID Fan with the inlet duct of the scrubber (or other AQC Equipment), a control damper in that duct to regulate the amount of gas that is recirculated and flow measuring devices to allow the control system to determine the amount of gas being recirculated and the total amount of gas flowing through the AQC system.
As the boiler load drops, the ID Fan throttles down the flow through the AQC equipment, lowering the discharge pressure of the fan and also diminishing the draft at the inlet of the AQC system. As a result very little differential pressure (or motive force) remains to drive the recirculation flow from the ID Fan outlet duct to the AQC system inlet duct making it impossible to maintain the required volumetric flow. The commonly accepted practice to remedy this problem is to install a booster fan in the recirculation duct to increase the flow and achieve the required recirculation flow rate. This fan is a large piece of equipment and would typically be driven by a motor as large as 500 to 2000 HP thus requiring a Medium Voltage switchgear starter, auxiliary bearing lubrication system, as well as various instruments for monitoring fan performance and status. The installed cost of this package could be substantial.
It would be advantageous to provide a more economical FGR system that does not require the addition of a booster fan, instead using the existing ID fan to provide the motive power for the recirculation gas flow.